An Optical Time Domain Reflectometer (OTDR) is a very important tool (or “function”) for characterization and diagnosis of optical links. The general concept of optical time domain reflectometry (also referred with the abbreviation OTDR), is to transmit an optical pulse into an optical link and measure a reflected signal from the link as a function of time. An optical pulse is more specifically known as a probe pulse, and also referred to as a pulse or an OTDR signal. In the context of this document, an optical link is generally an optical fiber communications link, simply referred to as a link. A reflected signal is also known as a returned or received signal, and in the context of this document, the three terms are used interchangeably, unless noted otherwise. Analysis of the reflected signal allows the physical properties of the link including the properties of the optical fiber, (generally referred to simply as the fiber) to be calculated. FIG. 1 shows an example of an OTDR diagram of the power of a returned signal over time. The physical properties of a link are shown in a diagram of the returned pulse as areas where the slope of the returned pulse is not smooth, and are known as “events”. The slope of the line 100 is two times the fiber loss per kilometer. The received optical signal (Rayleigh backscatter signal) is about 45 dB down from a launched 1 usec pulse. Examples of events on the link include, but are not limited to, bad splices 102, bends in the fiber (with too small a radius) 104, and flattened fiber 106. Section 108 shows an example that includes an angled fiber, low reflectance termination, or cut fiber.
The returned signal is derived from components including:
A. A Rayleigh backscattered signal, which arises from random scattering points distributed along the fiber. The returned signal, known as a backscattered signal, is the result of summation of a large number of tiny reflections generated along the length of the optical pulse. In a non-limiting example, if the probe pulse is 1 nsec (one nanosecond) in duration, the amount of fiber ‘lit’ by the pulse is 1/1,000 the amount of fiber lit by a pulse 1 usec (one microsecond) in duration. In this example, the return backscattered signal of a 1 nsec pulse is 30 dB down compared to a 1 usec pulse. Because the reflected signal equivalent to the pulse size is equal to a shorter length of fiber, a 1 nsec pulse enables higher spatial resolution of the optical fiber, facilitating greater accuracy (less uncertainty) of exactly where in the fiber the event occurs. In the current example, where a 1 usec pulse can provide a resolution of about 100 m (meters), a 1 nsec pulse can provide a resolution of about 0.1 m. For this reason, usually OTDR scans initially start with long pulse to quickly determine the general area of an event on the fiber, and then the scan ‘zooms’ in with a shorter pulse on specific events as required.
B. Reflections from discontinuities in the refractive index of the link, including, but not limited to splices, connectors, splitters, and fiber end faces. Reflections from these discontinuities have different characteristics than Rayleigh backscattering. For example, the amplitude of the reflected pulse is generally much larger than a backscattered signal and typically localized in a physical extent much shorter than the length of the optical pulse in the fiber. The reflected signal power is dependent on the power of the incident signal and the reflectance of the discontinuity (event) on the link that caused the reflection.
The OTDR function is an important tool to be used in Passive Optical Networks (PON). If a PON is used to provide the physical basis for a reliable communication network, the condition of the network and the installation should be accompanied with diagnostic tools ensuring this reliability. PON networks typically have long distances of optical fiber links (up to the order of tens of kilometers). The fiber links can be in rural areas, and are often buried, making locate failures or degradations of this physical plant a hard task. An OTDR function can be very important in locating points of failure in the link and understanding causes of degradation in the link.
An OTDR tool is used in PONs during the fiber installation to check the quality of the fiber, splices, and couplers, prior to, and during, the Optical Network Unit (ONU) installation (bring-up). OTDR tools can also be used during normal operation to confirm physical connectivity, to identify high loss fiber bends (such as due to physical damage, stress, or time degradation), detecting in-service degraded or failed ONUs, or to identify where fiber to ONU(s) has high loss prior to sending repair personnel. OTDR can also be used to detect out of service conditions (no communication with any/part ONU) and to identify where fiber has failed (cut or high bend loss).
Implementing OTDR in PONs is challenging for conventional OTDR test equipment. The PON as an optical network is generally characterized by a long fiber, an increased link budget due to the long fiber, and optical splits attenuating the signals. Usually the PON has a major splitting point (such as 1:8, 1:16, or 1:32 splitters being common) with additional minor splitting.
Referring to FIG. 2, a diagram of an EPON network, an OLT (optical line transmission equipment of the network provider) 200 communicates over a fiber optic network link (204, 206A, 206B, 206N) with ONUs (optical networking units associated with a user) (208A, 208B, 208N). A passive optical splitter 210 facilitates the OLT 200 communicating with the ONUs (208A, 208B, 208N). The one or more portions of a link from a splitter in the direction of the ONUs (in the current example, 208A, 208B, and 208N) are also known as “arms” of the link. The ONU 200 facilitates user connectivity, typically via a network switch 220, to a core network 222. A controller 224, also known as a host, can provide functions such as command, control, and monitoring of the PON. PONs are know in the art, and one skilled in the art will be able to choose a configuration of components, including but not limited to stand-alone or integrated, for a specific application.
As a result of the high loss from splitters and the fact that reflected signals see this loss in both forward and backward propagation, the OTDR signals are strongly attenuated, resulting in difficulty in seeing what happens after a split. In one non-limiting example, the optical attenuation (fiber, splitters, connectors, etc.) in each direction can be in the range of 30 dB, for a two-way loss of 60 dB. The reflection at the far end of the point can be −15 dB to −45 dB below the forward propagating OTDR probe signal, which means that a sensitivity level of −75 dBm to −105 dBm is needed at the receiver.
In addition, a PON has an additional complication—a forward propagating probe pulse from an OLT is split by a splitter and sent down all the splitter arms simultaneously. Each arm generates reflected signals back towards the OTDR. However, because the signal and backscatter reflections from all ONUs are combined by the splitter as the signals travel toward the OTDR, seeing small changes from events in one particular splitter arm is presents significant challenges. Even if an event is visible, discerning which arm an event occurs in is typically not possible.
Conventional solutions are divided into two broad categories. The first category of solutions is using stand alone OTDR test equipment. If there is a problem with a link, then the fiber for the link is disconnected from a network equipment, an OTDR test equipment is connected to the fiber in place of the network equipment, and the OTDR test equipment performs a test on the link. In the context of this description, a network equipment includes, but is not limited to network devices and user devices. If the problem with the link is thought to be on a side of the link closer to a network device, the OTDR test equipment can be connected in place of the network device, which in the case of PON is the OLT. If the problem with the link is thought to be on a side of the link closer to a user device, the OTDR test equipment can be connected in place of the user device, which in the case of PON is an ONU. In general, network devices are centrally located equipment providing services to a plurality of user devices, and user devices are equipment located at customer locations providing connectivity to a network device. Connecting an OTDR at different points in a network can be necessary due to the previously described difficulties in analyzing signals that traverse a splitter located between a network device and a user device. In these cases, the service is interrupted to the PON physical infrastructure, meaning that all users will lose service for the duration of the OTDR testing. Users are also referred to as customers, clients, and subscribers, depending on the context. In addition, this test requires an active intervention of a technician to physically facilitate the test. Because the network does not provide service during OTDR testing, the OTDR test equipment can use the same wavelengths used to provide service, referred to in this document as using the data signal transmission wavelength.
A second category of conventional solutions is based on using centralized OTDR test equipment, for example a stand-alone equipment using the 1625 or 1675 nanometer (nm) range, which is common in the industry, and referred to in current standards (such as ITU-T L.66) as maintenance wavelength. In the context of this document, the 1625 nm to 1675 nm range is referred to as 16XY. These wavelengths are used because these wavelengths can be wavelength division multiplex (WDM) separated from the PON wavelengths, and hence does not interfere with the PON services. Because the OTDR operates using different wavelengths than the wavelengths providing service to and from users, this technique is known as out-of-band, and service to the user(s) is not interrupted during OTDR. Industry standard communication wavelengths for providing service to/from a customer include the EPON/G-PON downstream (DS) signal (single fiber system) as specified in ITU-T G.984.2 and IEEE802.3ah as 1480 nm to 1500 nm and that of the EPON/G-PON upstream (US) signal as 1260 nm to 1360 nm. Other wavelengths for providing service to/from a customer include the 10GEPON/XG-PON downstream (DS) signal (single fiber system) as specified in ITU-T G.987.2 and IEEE802.3av as 1575-1580 nm range and that of the 10GEPON/XG-PON upstream (US) signal as 1260-1280 nm range. These downstream and upstream wavelengths are referred to in this document as 15XY and 13XY, respectively.
Current PON communication protocols for 1G (one gigabit per second) line rate include IEEE802.3ah-1GEPON (Gigabit Ethernet PON), FSAN (Full Service Access Network) and ITU-T G.984.1/G.984.2/G.984.3/G.984.4 GPON (Gigabit PON). The IEEE802.3av protocol for 10GEPON (10 Gigabit Ethernet PON) and ITU-T G.987.1/G.987.2/G.987.3/G.988 XGPON (10 Gigabit PON) are also known. The general concept of communication in PONs using the current communication protocols includes broadcasting from an OLT to ONUs using a downstream transmission at a first wavelength, and time division multiplexing (TDM) the upstream transmission from all the ONUs to the OLT using a second wavelength. The communications protocol manages and controls the media access of the different users.
The 1GEPON and 10GEPON control protocols are defined by the Multipoint Control Protocol (MPCP) given in the IEEE802.3ah (clause 64, 65) and IEEE802.3av clause (76, 77). The MPCP is packet based. Major MPCP concepts include time-stamping MPCP packets, sending grant packets in the downstream to indicate upstream transmission slots, sending report packets in the upstream to indicate reported data in queues and auto-discovery and a registration protocol. The GPON transmission control protocol (GTC) is defined in ITU G.984.3 and XGPON is defined in ITU G.987.3, and includes management done through a GTC header provided in a GTC frame.
1GEPON and 10GEPON also include a higher level control protocol, the Operation Administration and Maintenance (OAM) protocol defined in IEEE802.3ah (clause 57). The OAM protocol is also packet based. The GPON and XGPON higher level protocol is implemented by two types of messages—PLOAM messages and OMCI messages—defined in ITU G.984.3 and G984.4 and in ITU G987.3 and G.988 for XGPON.
Next generation access (NGA) protocols include the GPON next generation protocol marked as NGPON1, which includes XGPON1 (10G/2.5G) and XGPON2 (10G/10G).
FIG. 3 shows a conventional out-of-band OTDR system. Typically, OTDR test equipment 300 is stand-alone and expensive. Therefore, the OTDR test equipment is typically a shared resource. Sharing is typically accomplished using the above-described optical switches and WDM optical couplers 302. The OTDR test equipment is connected to a link only when there is a problem to the relevant link. An OTDR signal (probe pulse) 304 is sent from the OTDR 300 via the WDM 302 into the link 204. An OTDR signal (returned signal) 306 is received via the WDM 302 by the OTDR 300. If there is a problem after a splitter and the position is important then OTDR test equipment can be connected at an ONU. For example, OTDR 300 can be brought to the user location and substituted for ONU-1 208A, hence using fiber 206A for testing that arm of the link. In a further extension of this method, some operators add connectorized 16XY nm reflectors, typically fiber bragg grating (FBG) reflectors, at the ONU to reflect back the OTDR signal to allow the different paths from each ONU to be seen. The OTDR test equipment is typically located in a central office (CO) 308 or a nearby facility. Although a single PON is shown, typically the OTDR test equipment is connected to a plurality of PONs via an array of optical fiber switches and optical couplers under the control of a controller. This solution is not widely deployed because of the complexity and cost of this solution.
There is therefore a need for a system to perform analysis of links, including monitoring of links and providing smart alarms, in particular performing OTDR of PONs more frequently, while maintaining service for users, and at a lower cost than conventional solutions.